[llvm] fix `llvm.fma.f16` double rounding issue when there is no native support (PR #171904)

[Folkert de Vries via llvm-commits]

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folkertdev wrote:

Well then I'm not sure what your suggestion is here? I don't see how we can implement anything valid for `bf16`?

https://github.com/llvm/llvm-project/pull/171904